1

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: March 08, 2021, 06:36:32 PM »

thank you a lot!

2

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: March 02, 2021, 12:35:05 PM »

TP2 and TP8 have both the hum with the same frequency.
The Voltage is different.

TP2: +-0,8V AC
TP8: +-0,08V AC

3

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 27, 2021, 11:06:38 AM »

thank you -  i always connect via balanced cables.

So, do i get this right, if the hum occurs the first time on TP2, according to your schematics it's very likely that the tube is causing the noise because it sits between TP1 and TP2?

4

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 22, 2021, 11:30:26 PM »

and I just discovered that the hum Signal is only on the + output on the xlr connector. not on the - pin. if I switch polarity, this is inverted.

5

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 22, 2021, 11:00:49 PM »

Thank You.

TP1 = nothing
TP2 = strongest noise Signal (+- 0,8V AC)
TP3= +-0,01V AC
TP4= +-0,005V AC (although i cannot interprete that signal good enough on the scope)

All noise signals are sine waves with approx 80hz.

6

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 20, 2021, 01:16:45 PM »

oh, ok, i feel a bit releaved.
I may borrow an oscilloscope next week, can u tell me the points that i should check? since i'm not that familiar with oscilloscopes - i'll have someone who will help here!
I also found out that the hum-frequency changes when i move the gain switch (not the knob) (lowest frequency at lowest gain) and that it also appears when the preamp is not inside the housing.

7

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 18, 2021, 04:47:21 PM »

I may borrow an osci soon. Meanwhile, i looked at your picture in the assembly guide and then at the preamp and I realised I may have mounted the output transformer upside down in my build. could that be the cause of my troubles or could it have ruined any other component?

pic is https://drive.google.com/file/d/1NDLS8cf43oms7t91bm7AYNvzPSeoEEoV/view?usp=sharing

8

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 15, 2021, 11:50:11 AM »

So I tested at another Housing - the same effect happens. there's also some microphonic noise at the tube. 

If I were to change tube - which model should I chose?

9

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 01, 2021, 05:50:05 PM »

Ok I hope I'm gonna be able to do that in a few days - from now I can say, that the hum starts around 7 minutes after powering on the unit - before that it works perfectly. 
I've recorded it for a few seconds and put it as a wetransfer download link here. 

https://we.tl/t-nCujNYa6M1

10

MP566 Tube microphone preamplifier / Re: Hum at 60hz

« on: February 01, 2021, 05:29:02 PM »

Thank you for the fast answer.

I've built the housing also from SoundSkulptor (501) - The Power Transformer is more than a meter apart. I don't know about the specs of that one but from the reading of it it says "switching".

SO I guess trying another housing might be my first step?

11

MP566 Tube microphone preamplifier / Hum at 60hz

« on: February 01, 2021, 01:14:16 PM »

I've recently built an MP566 and since a few weeks, I#m getting strong hum at 60-70hz.

The hum is also there if there's no input connected.
Without the module, there is no hum coming from the single slot housing.
The gain and input knobs do not change the volume of the hum.
the output pot changes it.
it's not there during the first minutes when I power on the device.
when directly connected to my AD converter (SSL Alphalink) this hum-noisefloor is at -60dBFS.

Any help is very appreciated!
Merci beaucoup!

Christoph